Identification and recognition

Illumination

Illumination greatly affects the ability to identify persons or objects. Shadows, high contrast and backlit scenes all make identification and recognition more difficult.

At distances of 15-20 m you will need a 50 mm lens to ensure that a face covers approximately 80 pixels on the horizontal plane. However, positive identification cannot always be guaranteed at the 100-150 lux illumination typically found in an office corridor or subway station. Camera features such as WDR Mode and sensors that perform well in low-light situations can help, but the best results will be obtained if these features are combined with additional lightning and the relocation of cameras to avoid backlit situations.

In outdoor surveillance it is important to remember that sunlight varies in intensity and direction over the course of a day. Weather conditions will also affect lighting and reflection. Snow, for example, will intensify the reflected light, while rain and wet tarmac will absorb light. For identification of a human face, balanced illumination in the region 300-500 lux is recommended. For license plate identification, 150 lux may be sufficient.

Figure 3: Examples of how light conditions affect identification. Image A) has a lux level of 1600 with a favorable light direction. B) has 350 lux, with backlight. C) has 7 lux, with a favorable light direction. D) has a lux level of 1.5.

In low light conditions, camera sensors produce significant amounts of noise that can affect the image, making identification more difficult. There is always a trade-off between noise, shutter speed, and depth of field at any given level of illumination, where better lighting conditions allow you to improve all of these.

Color is often an important factor for identification. To ensure color fidelity, the camera’s white balance should be adjusted to suit the color temperature of the light source(s) used. In outdoor surveillance, the color temperature will change over the course of the day, requiring automatic white balancing to maintain color fidelity.